GRPCHost now has a property channelCreds which is used when creating a GRPCChannel.

of the proto if desired, or just goes to pick_unused_port_or_die if not
specified

reset option to the mark, create a closed loop config params (empty
message) for consistency with other tests.

This allows us to get back to single binary tests where appropriate, which will help in-depth profiling efforts.
I've built this atop my smoke_test changes as they inspired me to get this done.